1. Field of the Invention
The present invention relates to symbol boundary detection in an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) receiver.
2. Background Art
Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc. In particular, the IEEE Standard 802.11a, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band”, specifies an OFDM PHY for a wireless LAN with data payload communication capabilities of up to 54 Mbps. The IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Hence, the IEEE Standard 802.11a specifies an OFDM PHY that provides high speed wireless data transmission with multiple techniques for minimizing data errors.
A particular concern in implementing an IEEE 802.11a based OFDM PHY in hardware involves providing a cost-effective, compact device that can be implemented in smaller wireless devices. Hence, implementation concerns typically involve cost, device size, and device complexity.
FIG. 1 is a diagram of a preamble 10 used by an OFDM receiver for synchronization with an 802.11 OFDM packet 12, reproduced from FIG. 110 (Section 17.3.3) of the IEEE Standard 802.11a. In particular, the preamble 10 is a Physical Layer Convergence Procedure (PLCP) preamble having a first training portion (i.e., a short preamble) 14 and a second training portion (i.e., a long preamble) 16. The first training portion 14, typically used for signal detection, automatic gain control, diversity selection, coarse frequency offset estimation, and timing synchronization, includes ten (10) identical short preamble symbols (t1, t2, . . . t10) 18; each short preamble symbol 18 is implemented as a 16-sample symbol. The second training portion 16 includes long training symbols (T1 and T2) 20 and a guard interval (GI2) 22. The second training portion 16 typically is used for channel and fine frequency offset estimation.
A particular concern involves accurate detection of the symbol boundary 24 by the OFDM receiver. Accurate detection of the symbol boundary 24 is critical for accurate Fast Fourier Transform (FFT) processing of the long training symbols 20 and subsequent symbols 26 following the preamble 10. One technique for detecting the symbol boundary 24 is to perform autocorrelation of the short preamble symbols 18 for generation of an autocorrelation power signal: since the short preamble symbols 18 should be identical, the resulting autocorrelation power signal should have a maximum value until the end of the first training portion 14, at which point the autocorrelation power signal drops after no further short preamble symbols are detected after the last symbol (t10).
Hence, the symbol boundary 24 typically can be detected by supplying the autocorrelation power signal to an edge detection circuit which can detects the symbol boundary 24 in response to detecting the falling edge of the autocorrelation power signal passing below a set threshold.
However, an arbitrary setting for the threshold may greatly affect the accuracy of the symbol boundary detection. In particular, each received packet may originate from a different wireless source; hence, the energy in each received packet may vary; moreover, each received packet may have encountered different wireless channel characteristics (e.g., fading), and hence have different noise characteristics. Consequently, the energy and noise characteristics will vary with each packet, affecting the time in which the corresponding autocorrelation power signal will pass below the set threshold.